The invention relates to a membrane, especially a gas separation and pervaporation membrane based on graft copolymers.
The inventive membrane serves to separate gas mixtures and/or gas mixtures that are comprised of gases and vapors of organic solvents, and/or for pervaporation of aqueous/organic or organic/organic mixtures.
It is known to use for the separation of gas mixtures and vapors dense films of organic plastic materials which have the function of a membrane. In general, two types of membranes are known, i.e., a) integral asymmetric membranes and b) composite membranes.
Integral-asymmetric membranes are comprised of polymers or compatible polymer mixtures that are brought into the form of a self-carrying and supporting membrane by a phase inversion process. Accordingly, it is possible to achieve a very thin, stable separating layer of the polymer or the polymer mixture.
Composite membranes are comprised of thin films that are applied onto suitable support of materials of organic or inorganic substances. The support materials must be stable with respect to their shape and must allow for incorporation into modular systems. They must provide a high gas permeability. The supports can be selected in the form of polymer, organic materials, for example, from microporous polysulfone, polypropylene, polyvinylidene fluoride, or polyetherimide supports and in the form of inorganic materials as microporous glasses or microporous aluminum oxide. Both organic and inorganic support materials can be treated for smoothing the surface with a very thin film of an especially gas permeable polymer, for example, polydimethylsiloxane or polytrimethysilypropine.
The actual separating layer is applied to the surface of the treated or untreated microporous carrier whereby for the purpose of allowing great gas flows during the gas and gas/vapor separation it is desirable to apply a film of the separating polymer that is as thin as possible, for example, 0.5 to 3 .mu.m. With especially thin films free of micropores the gas flow through the separating layer however may be so great that the resistance of the support material has an effect on the separating efficiency of the membrane (I. Pinnau, J. G. Wijmans, I. Blume, T. Kuroda, K.-V. Peinemann, Gas Permeation through Composite Membranes, J. Membrane Sci., 37 (1988) 81 and U.S. Pat. No. 4,931,181). As a rule of thumb it can be presupposed that the effect of the support resistance on the selectivity is negative when the flow of gas through the separating layer is in the range of approximately 10% of the flow through the support. The efficiency of the entire membrane thus depends on the adjustment of the support material and the actual separating layer relative to one another. Not every support material, for example, PVDF supports, polysulfone supports, and especially hollow fiber membranes, can be manufactured so as to have the required gas flows in order to provide an optimal basis for very quick separating materials, as, for example, a PDMS membrane of 0.5 to 1 .mu.m thickness.
For a special application in the separation of vapors under high pressure, and especially in pervaporation of organic/organic or aqueous/organic solutions it is advantageous to use thicker polymer films of approximately 3 .mu.m up to 100 .mu.m. An increased feed pressure is, for example, advantageous for the membrane process of a gas/vapor separation for cleaning exhaust air because this achieves, on the one hand an increased flow through the membrane can be achieved and, on the other hand, the ratio of feed pressure to permeate pressure can be adjusted in a cost efficient manner.
The separating efficiency of a given polymer will decrease more or less with an unfavorable pressure ratio.
Known polymers that are suitable for membrane separating processes are the following:
polydimethysiloxane PA1 poly(4-methyl-1-pentene) PA1 ethylcellulose PA1 natural rubber PA1 L D polyethylene PA1 cellulose acetate PA1 providing a base component comprising a first polymer of a repeating unit of the following general formula: ##STR7## wherein m=0.1-0.9, n=0.9-0.1, p=0.03-0.04, R.sub.1 can be a linear, branched or cyclic C.sub.1 -C.sub.12 hydrocarbon radical, R.sub.2 can be a linear, branched or cyclic C.sub.1 -C.sub.12 hydrocarbon radical, and wherein at least one of said R.sub.1 and R.sub.2 is a linear or branched hydrocarbon radical with a terminal C.dbd.C double bond; PA1 dissolving the first polymer in a solvent to form a polymer solution; PA1 adding a copolymer component to the polymer solution; PA1 adding a cross-linking catalyst to the polymer solution; PA1 partly cross-linking the first polymer with the copolymer component to a desired degree to form a partly cross-linked graft copolymer solution; PA1 applying the partly cross-linked graft copolymer solution to a support; and PA1 continuing cross-linking until the graft copolymer is insoluble. PA1 providing a first polymer of a repeating unit of the following general formula: ##STR8## wherein m=0.1-0.9, n=0.9-0.1, p=0.03-0.04, R.sub.1 can be a linear, branched or cyclic C.sub.1 -C.sub.12 hydrocarbon radical, R.sub.2 can be a linear, branched or cyclic C.sub.1 -C.sub.12 hydrocarbon radical, and wherein at least one of said R.sub.1 and R.sub.2 is a linear or branched hydrocarbon radical with a terminal C.dbd.C double bond; PA1 providing a second polymer containing C.dbd.C double bonds; PA1 dissolving said first polymer and said second polymer in a solvent to form a polymer solution; PA1 adding a copolymer component to the polymer solution; PA1 adding a cross-linking catalyst to the polymer solution; PA1 partly cross-linking the first polymer and the second polymer with the copolymer component to a desired degree to form a partly cross-linked graft copolymer solution; PA1 applying the partly cross-linked graft copolymer solution to a support; and PA1 continuing cross-linking of the graft copolymer until the graft copolymer is insoluble. PA1 a) Membranes of solutions according to Example 1 (No. 1-11): PA1 a) Membrane test pieces PA1 b) Composite membranes on microporous polyethermide supports (PEI). PA1 nitrogen (N.sub.2), oxygen (O.sub.2), chloromethane (CH.sub.3 Cl), and chloroethane (CH.sub.3 CH.sub.2 Cl) were measured.
In practice the elastomer polydimethylsiloxane (PDMS) has been proven to be the most used membrane material for the above described applications for problem solving.
The manufacture of composite membranes made of PDMS is for example disclosed in EP-A 0 254 556 and EP-A 0 181 772.
With respect to vapor separation it has been demonstrated (See K.-V. Peinemann, J. M. Mohr, R. W. Baker, The Separation of Organic Vapors from Air, AIChE Symp. Ser., 250 (1986) 19) that the technical suitability of a given membrane material is effected by two important parameters: the membrane selectivity and the ratio of feed pressure to permeate pressure. The possible pressure ratio for a membrane process is substantially determined by the separation task to be performed and must be adjusted according to economical criteria. The membrane selectivity is primarily determined by the membrane material.
In order to solve a given separation problem the required membrane surface area is greater for the same flow for a less selective membrane as compared to a more selective membrane. The flow of a membrane depends directly on its thickness. The membrane surface area thus can be reduced, when it is possible to provide thinner membranes. However, the increase of flow is limited because films thinner than 1 to 3 .mu.m can be produced with large surface areas that are free of defects only under great difficulty. On the other hand, for very large flow rates through the composite membrane the resistance of the support material has a negative influence on the selectivity. Under these circumstances, the expected selectivity is not achieved even with a pore-free membrane. The increase of the flow rate thus does not have the desired effect.
A further issue is to be considered for the use of membrane separation processes in the recycling process of solvents from exhaust air because a given membrane with the permeability pO.sub.2 /pN.sub.2 of 2, compared to a membrane of a higher separating factor for solvent vapors but same separating factor for O.sub.2 /N.sub.2, enriches oxygen within the device in large amounts. A more selective membrane thus reduces possible safety risks of a membrane separating device.
It is an object of the invention to provide a membrane with a higher selectivity. Furthermore, it is an object of the invention to provide polymers which can be made into a membrane film and which adhere well and permanently to a microporous support.